As reported here, the JRELoadError probably happens on case sensitive systems. To fix it, click on the application icon of Sweet Home 3D while maintaining the ctrl key pressed to display its contextual menu, choose Show Package Contents item, open the Contents folder and rename the folder [font=courier new]Plugins[/font] as [font=courier new]PlugIns[/font] (with an uppercase [font=courier new]I[/font]).
This issue will be fixed in versions > 5.4.1.